1. Field of the Invention
The present invention relates to audio signal processing, and, in particular, to acoustic beam forming with an array of microphones.
2. Description of the Related Art
Microphone arrays can be focused onto a volume of space by appropriately scaling and delaying the signals from the microphones, and then linearly combining the signals from each microphone. As a result, signals from the focal volume add, and signals from else where (i.e., outside the focal volume) tend to cancel out.
One of the problems with a simple linear combination of signals is that it does not address the situation when noise occurs at or near one of the microphones in the array. In a simple linear combination of signals, such noise appears in the resulting combined signal.
These is prior art for canceling noise sources whose positions are known, such as those based on radar jamming countermeasures, where the delays and scales of the different microphones are adjusted to produce a null at the known position of the noise source. These techniques are not applicable if the position of the noise source is not well known, or if the noise is generated over a relatively large region (e.g., larger than a quarter wavelength across), or in a strongly reverberant environment where these are many echoes of the noise source.
Other prior art techniques for noise suppression, such as spectral subtraction techniques, operate in the frequency domain to attenuate the signal at frequencies where the signal-to-noise ratio is low. In the context of acoustic beam forming, such techniques would be applied independently to individual audio signals, either before the signals from the different microphones are combined or, after that combination, to the single resulting combined signal.